Create a plane that always fits the camera viewport
There is a plugin by tobbew to do exactly what you ask: create camera image plane

"it creates a plane and sets up some drivers so when the distance from the camera (-z) is changed the plane is automatically scaled to cover the screen and gives it a cycles material so i'll just have to load the texture"

Once the plugin is installed, all you do is select the camera and run the plugin (press the space bar and start typing camera):



And it creates a plane in front of the camera. If you move the camera the image plane will always follow it. If you move the plane away or closer to camera it will also rescale automatically.


here's the blenderartists.org thread for additional info.